1. Field of the Invention
This invention relates to controlled release nitrogen fertilizers and methods for their preparation. More particularly, the invention relates to a ureaformaldehyde fertilizer composition and a method for producing these compositions exhibiting increased conversions of urea nitrogen to cold water insoluble nitrogen (CWIN) with low conversions to hot water insoluble nitrogen (HWIN). The new method relates to the discovery that ammonia nitrogen in relatively small amounts in the ureaformaldehyde reaction inhibits the polymerization to HWIN compounds and allows the reaction to CWIN to occur in short reaction times at elevated temperatures. A new and effective aminoureaformaldehyde fertilizer composition, combining high CWIN conversions, high nitrogen availability indices (AIs), and low unreacted free urea concentrations, is provided by the new method.
2. Description of Related Art
Ureaformaldehyde condensation products are widely used as slow release nitrogen fertilizers in crops and ornamental plants. The condensation products normally contain methylene urea polymers which have limited water solubilities and release nitrogen throughout an extended period. The methylene urea polymers are understood to be degraded by microbial action into water soluble mineral nitrogen slowly. The ureaformaldehyde fertilizers are usually evaluated by the amount and the release characteristics of their water insoluble nitrogen.
The longer chain water insoluble ureaformaldehyde polymers are more slowly available than the shorter chain polymers, and it is generally known in the art that the efficiency of nitrogen release decreases as the methylene urea chain lengths increase. When methylene urea chain lengths are long enough that their nitrogen contents are insoluble in hot water, it is known that the nitrogen does not become available for plant utilization in soil for a very long period of time, if ever.
Many of the granular ureaformaldehyde fertilizers now found in commercial use have 60 percent or more of their CWIN insoluble in hot water, and practically unavailable as a plant nutrient.
The prior art has decreased the ratio of HWIN to CWIN in ureaformaldehyde fertilizers by admixing additional water soluble urea during the preparation, but this process leaves large amounts of free and unreacted urea and increases the phytotoxicity of the mixture.
Slow release ureaformaldehyde fertilizers are usually prepared by reacting urea and formaldehyde at elevated temperatures in an alkaline solution to produce methylol ureas. This reaction mixture is then acidified to polymerize the methylol ureas to methylene ureas which increase in chain length as the reaction is allowed to continue.
Greidinger et al disclose in U.S. Pat. No. 4,089,899 the reaction of urea and formaldehyde in the presence of an acid catalyst for long reaction times at low temperatures to prepare slow release ureaform compounds with lower average degrees of polymerization than conventional fertilizers. The Greidinger products contain large amounts of free urea which are undesirable from the standpoints of rapid nitrogen release and plant phytotoxicity.
Formaini disclosed in U.S. Pat. No. 3,677,736 a multistage method of producing liquid fertilizer suspensions of ureaformaldehyde with a high Activity Index (AI), by reacting urea and formaldehyde in the presence of ammonia first under alkaline conditions and then under acid conditions. Conversions of urea nitrogen to CWIN were low, leaving large amounts of unreacted urea.
Goertz disclosed in U.S. Pat. No. 4,378,238 a particulate controlled release solid fertilizer, the reaction product of urea and formaldehyde, which contained polymeric nitrogen in the form of methylene urea polymers of varying chain lengths with at least 60 percent of the polymeric nitrogen in the form of cold water soluble nitrogen polymers (CWSNP) and, preferably, less than 20 percent CWIN. The Goertz product contains short chain polymers selected from the group consisting of methylene diurea and dimethylene triurea. Goertz teaches the use of a degree of polymerization greater than 1.5, where the degree of polymerization is the ratio of urea to methylene groups in the methylene urea polymers. The conversion of urea nitrogen to CWIN was generally low, which indicates that most of the nitrogen is quickly released when applied to the soil in substantial amounts thereby creating conditions phytotoxic to plants.
Edwin F. Hawkins claimed a clear liquid ureaformaldehyde based fertilizer, in U.S. Pat. No. 4,554,005, containing 30 percent triazone, less than 2 percent methylene diurea, and less than 3 percent monomethylol urea. The liquid was made by reaction of urea and formaldehyde at mol ratios between 1.2 and 1.6/1 with ammonia amounting to between 3 and 3.5 percent by weight of the reaction mixture at a pH between 8.8 and 9.2, maintained by continued addition of caustic solution and then continuing to heat after terminating the caustic addition. Although Hawkins used ammonia to produce a storage stable clear liquid fertilizer, the product contained no CWIN and did not provide a slow release, nitrogen source. Although Hawkins did provide a method of producing a liquid with reduced phytotoxicity, he did not find or suggest a method for producing a granular product whose nitrogen was composed substantially of safe and efficiently used CWIN and HWSN.
The prior art provides processes for ureaformaldehyde fertilizers exhibiting high activity indices (AI) with low conversions of urea nitrogen to WIN, and also processes for fertilizers exhibiting high conversions to WIN, with low AIs. No methods have been provided, or proposed, for the preparation of ureaformaldehyde fertilizers exhibiting high conversions of urea N to WIN and high AIs.
No granular ureaformaldehyde fertilizer compositions have been taught in the prior art which exhibit the properties of high AI and high urea conversions to CWIN, although the agronomic desirability of such a product has been known for a long time.
Semi-spherical granules have been produced commercially and reported in the art by a variety of techniques using water soluble nitrogen products, such as urea, potassium nitrate, and ammonium phosphate, and the practical advantages of handling, blending, and storing the semi-spherical granules are known and well documented. The prior art has not provided methods for the direct production of water insoluble ureaformaldehyde fertilizers in the form of attrition resistant semi-spherical granules.
Terms used herein are defined as follows:
CWSN=cold water soluble nitrogen; PA1 CWSNP=cold water soluble nitrogen polymers; PA1 CWIN=cold water insoluble nitrogen; PA1 HWIN=hot water insoluble nitrogen; PA1 HWSN=hot water soluble nitrogen; ##EQU1## Highly Available CWIN=AI of 50-85; Semi-Spherical=free flowing, generally ball shaped; PA1 Fertilizer Enhancing Ingredients=materials which improve nutritive or physical properties of solid fertilizers; PA1 Percent=weight percent; unless otherwise specified.
An aminoureaformaldehyde polymer is a condensation product of ammonia, urea, and formaldehyde. Its formation may be represented by the following equation: ##STR1##
Aminomethylene dimethylene triurea, a typical condensation polymer of 1 mol formaldehyde, 1 mol urea, and 0.33 mols of ammonia may be represented in many isomeric forms but may be simply illustrated by the following linear structural formula: ##STR2##